Effective Diffusivity Of Water Vapor Research Articles

Development of a differential photoacoustic system for the determination of the effective water diffusion and water vapor permeability coefficients in thin films

This paper focused on developing a methodology and metrology using a differential photoacoustic (PA) system to determine the effective water vapor diffusion coefficient (Deff) and the effective permeability coefficient (Π) in thin films as a piece of paper and standard polystyrene for a controlled relative humidity. The methodology proposes a new differential photoacoustic system, including the water reservoir, relative humidity, and temperature detectors. Two cells, reference/sample, were used to obtain the instrumental function to reduce the electronic and environmental noises. A method based on the study of ln[1−(S−S0)/ΔS]=t/τD and the behaviors of R2 as a function of the number of data was proposed to assess the region in which the photoacoustic signal should be processed to determine each effective coefficient. S is the amplitude of the PA signal, S0 is the initial amplitude value, ΔS is the change, t (time), and τD is the water vapor diffusion time. The effective water diffusion coefficient (Deff) for water and polystyrene was 1.90 × 10−11 m2/s and 3.09 × 10−11 m2/s, respectively. The permeability coefficient value for the piece of paper was 4.18 × 10−9 mol kg−1 cm−2 s−1 Pa−1, while for polystyrene, it was 6.80 × 10−9 mol kg−1 cm−2 s−1 Pa−1 for 70% of relative humidity. This methodology can be extended by changing the moisture content on the chamber to obtain the dependence of Deff as a function of relative humidity.

Investigation on Water Vapor Adsorption of Silica-Phosphonium Ionic Liquids Hybrid Material.

Adsorption and diffusion of water vapor in phosphonium ionic liquid modified silica gel were studied, aiming to reduce the loading of water vapor in porous materials. The modified silica gel was prepared through a grafting method and characterized by FTIR, thermal gravity analysis and X-ray photoelectron spectroscopy. N2 sorption isotherms at −196 °C and CO2 sorption isotherms at 0 °C were also measured to analyzee the porosity. Water vapor adsorption equilibriums at 25 °C up to 30 mbar were tested. The results indicate that the ionic liquids (ILs) phase acts as a protecting film which decreases water vapor adsorption. The improvement of water-resistant performance is also attributed to the decrease of micro-porosity and silanol groups on the silica surface. Diffusion behavior of water vapor on modified silica was determined on the basis of the adsorption equilibrium. The effective diffusivity of water vapor in modified silica is almost the same as in bare silica and decreases with the increasing of water vapor loading.

Numerical Analysis of Thermal and Moisture Bridges in Insulation Filled Masonry Walls and Corner Joints

In recent years, thermal insulation filled masonry blocks have become widespread in Central-Europe. These blocks can satisfy thermal performance requirements without the need of additional insulation. However, these requirements in the building regulations only consider thermal, but neglect moisture aspects. This paper presents a comparative analysis of steady-state numerical conjugated heat- and moisture transport FEM simulations of masonry walls. The hygrothermal material properties of the insulation filled masonry blocks were measured in laboratory. In the paper, besides a wall section, a wall corner joint is presented, both modeled in 2D from complex building elements, such as insulation filled blocks, and were tested using different fillers (aerogel, expanded perlite, expanded polystyrene, mineral wool and PUR foam), respectively. Monthly variation of the fillers’ thermal conductivity, thermal and moisture transmittance and effective water vapor diffusion resistance of the walls, as well as linear thermal and moisture transmittance of the wall corner joints were examined in details. A comparison between detailed and simplified modeling were also carried out. The evaluation of the results shows that there are noticeable differences in trends between thermal and moisture transmittances, latter show significantly greater variation and depends mainly on the hygrothermal behavior of the filler. Based on effective water vapor resistance factors, we showed that assuming the same vapor transmission properties for all type of filled masonry blocks is a mistake.

Humidity regulation by stretched PP and PLA films with dispersed CaCl2

ABSTRACTPolymer materials that regulate the relative humidity in their environment are relevant for applications in the packaging and building sectors. By integration of salts in polymer structures, such materials are able to absorb and desorb high amounts of water vapor. In this study, films of polylactic acid and polypropylene with dispersed calcium chloride (2 and 4 wt %) were produced and biaxially stretched to induce the formation of cavities. The resulting cavities in these films account up to 10 vol % and are able to contain emerging calcium chloride solution formed by water vapor absorption. These films absorb reversibly up to 15 wt % water vapor at 75% relative humidity at 23 °C. This absorption behavior is described by effective diffusion and effective sorption coefficients. Using a simple model, the effective water vapor diffusion coefficient of these films can be estimated from the permeation coefficient of the polymer and the sorption coefficient of the absorber. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45713.

Model-based investigation into atmospheric freeze drying assisted by power ultrasound

Atmospheric freeze drying consists of a convective drying process using air at a temperature below the freezing point of the processed product, and with a very low relative humidity content. This paper focuses on the use of a simple one-dimensional model considering moving boundary vapor diffusion to describe the ultrasonic assisted atmospheric freeze-drying of foodstuffs. The case study is the drying of apple cubes (8.8mm) at different air velocities (1, 2, 4 and 6m/s), temperatures (−5, −10 and −15°C), without and with (25, 50 and 75W) power ultrasound application. By fitting the proposed diffusion model to the experimental drying kinetics, the effective diffusivity of water vapor in the dried product was estimated. The model was successfully validated by drying apple samples of different size and geometry (cubes and cylinders). Finally, a 23 factorial design of experiments revealed that the most relevant operating parameter affecting the drying time was the applied ultrasound power level.

Understanding the gas diffusion layer in proton exchange membrane fuel cells. I. How its structural characteristics affect diffusion and performance

The proton exchange membrane fuel cell (PEMFC) has a significant potential in transportation, backup, and portable power applications, although there still are remaining technical and cost challenges. A key current goal is improving the performance while reducing the cost of the gas diffusion layer (GDL). Designing a commercial GDL, however, is far more complex than simply making a porous, sturdy, conductive layer, because of the trade-offs among performance, manufacturability, and cost. An improved understanding of its multifarious functions in the fuel cell can help attain this goal. Here, we identify 11 key characteristic parameters of the GDL and their significance to its performance. We begin a discussion of some of these parameters in this paper, specifically those related to the structure of the GDL substrate and the microporous layer (MPL), how these are measured experimentally ex-situ, how they influence fuel cell performance, and how they can be altered via the manufacturing process. In particular, we investigate the correlation between ex-situ measured effective diffusivity of water vapor and in-situ performance and limiting current density in a PEM fuel cell. Further, we examine the effect of adding multiple MPLs, MPL loading, and MPL particle size on cell performance under both wet and dry operating conditions.

F202 伝熱面一体型吸着体の平衡吸着量および脱着速度(OS10 外燃機関・廃熱利用技術(1))

Enhancement of adsorption/desorption rate in an adsorption heat exchanger is effective to downsize adsorption chillers and desiccant systems Exothermic/endothermic phenomena during adsorption/desorption decrease with the adsorption/desorption reaction rate, so that it is necessary to improve the heat transfer performance of adsorption heat exchanger. Thin film adsorbent coated on the plate is promising to enhance the heat transfer, because it has large contact area on the plate. In the previous study, it was clarified that the heat transfer performance of a porous alumina thin film as an adsorbent coated on an aluminum plate was greater than that of conventional particle packed bed of activated alumina. In this study, the adsorption equilibrium and the desorption rate of porous alumina thin film were measured. The parameters in BET equation were determined and Polanyi's potential theory was examined. In addition, effective diffusivity of water vapor in desorption process was summarized.

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.

Dehydration of Ethanol-Water Azeotropic Mixture by Adsorption Through Phillipsite Packed-Column

Phillipsite is a natural zeolite material available in several locations in Jordan. Phillipsite with and without treatment was used for dehydrating ethanol in the adsorptive distillation process. Molecular sieves 4A was taken as a reference material. According to the breakthrough curves phillipsite treated with 1 M calcium chloride solution gave the best performance. The water uptake using the 1 M-phillipsite was 0.1054 g H2O/g adsorbent while the water uptake using the 4A molecular sieves was 0.1030 g H2O/g adsorbent. The Guggenheim, Anderson, and De Boer (GAB) model and the Frenkel, Halsey, and Hill (FHH) model were used to represent the isotherms for water sorption on the adsorbents used. The GAB model fit the experimental data relatively better than the FHH model for all cases, except for the case of using 2 M-phillipsite. Using the Crank diffusion model the effective diffusivity of water vapor in raw phillipsite is 10.40 × 10−8 mm2s−1 while it is 6.9 × 10−8 mm2s−1 in the 1 M treated phillipsite.

The Effect of Relative Humidity on Binary Gas Diffusion

The dependence of diffusion coefficient of O2-N2 mixture in the presence of water vapor was experimentally determined as a function of relative humidity (RH) with different temperatures using an in-house made Loschmidt diffusion cell. The experimental results showed that O2-N2 diffusion coefficient increased more than 17% when RH increased from 0% to 80% at 79 degrees C. In the experiments, the RH in both top and bottom chambers of the diffusion cell were the same, and the pressure inside the diffusion cell was kept as ambient pressure (1 atm.). Maxwell-Stefan theory was employed to analyze the mass transport in the diffusion cell, and found that there was no effective water vapor diffusion taking place, indicating that the gas diffusion in this ternary (O2-N2-water vapor) system could be considered binary gas (O2-N2) diffusion. The Fuller, Schettler, and Giddings (FSG) empirical equation of the kinetic theory of gases was generalized to accommodate the dependence of the binary diffusion coefficient on the RH. The prediction of the generalized equation was found to be consistent with experimental results with the difference of less than 1.5%, showing that the generalized equation could be applied to calculate the diffusion coefficients of the binary gaseous mixture with different temperature and RH values. The effect of water vapor on the increase of O2-N2 diffusion coefficient was discussed using molecule theory.

An Empirical Model for the Flooding Behavior of Ultra-thin PEM Fuel Cell Electrodes

A quantitative empirical model is applied to explain the onset of cathode flooding behavior of PEM fuel cell membrane electrode assemblies having the ultra-thin 3M NSTF (nanostructured thin film) electrocatalysts and first generation Toray carbon paper gas diffusion layers (GDL). An empirical method is used to define the current density at the onset of cathode flooding, and calculated vapor phase water removal rates at specific water balance conditions are found to give excellent quantitative agreement with the measured rates of water generation for three different first generation GDL's. Effective water vapor diffusion coefficients porosites and saturation levels for the GDL's can be extracted from the water balance data at the in-situ operating conditions. Application of the method to later generation GDL's for which the flooding occus at much lower temperatures is also discussed, with the conclusion that liquid phase transport plays a greater role for these GDL's.

Water and Water Vapor Penetration Through Coatings

Water and water vapor transport in two-layer systems where one layer acts as water retarder are analyzed. In the experimental work, four external paints supposed to protect the underlying structure against water under common service conditions are applied on glass fiber reinforced concrete substrates. The water absorption coefficient and the effective water vapor diffusion coefficient are chosen as evaluation parameters for the assessment of water and water vapor transport properties of the systems. In the water absorption experiments, two consecutive quasi-steady regimes are identified, the first with an almost constant water flux into the system, and the second obeying the square-root-of-time rule for cumulative mass of water in the system. The water protective qualities of all studied paints can be considered as satisfactory for common service conditions. The effective diffusion coefficients of all systems are found to be within the error range of the measuring method compared to the substrate itself, which is a very positive feature for external paints.

Effective water vapor diffusion coefficient of snow under a temperature gradient

The wave‐like temperature and density distributions in snow under an applied temperature gradient, found by Sokratov and Maeno [1998], were used to estimate the effective water vapor diffusion coefficient in snow. Values obtained were (0.6−2.0)×10−5m2 s−1, lower than the water vapor diffusion coefficient in air and much smaller than previous results found in the literature. The difference was discussed with the use of results of CO2 diffusion measurements in snow under an isothermal condition and the introduction of a new parameter, the gradient enhancement factor (ƒ). It was concluded that the effective water vapor diffusion coefficient of snow is dependent on the porosity, tortuosity (т), and gradient enhancement factor. The mean values of т and ƒ were 1.73 and 2.3, respectively, for the snow studied.

Heat transfer by vapor transfer in ventilated snow

A method has been developed for determining the heat transfer due to vapor transfer in snow through which air is flowing in a direction parallel but opposite to the heat flow. The experimental technique consists in determining the effective diffusivity of water vapor through snow, De, which was subsequently used to calculate the extent of heat transfer due to vapor diffusion. The results are interpreted as, kv equivalent thermal conductivity due to vapor transfer, and can be represented by kv=0.523 Ls DeRT e0.0857 t where Ls, represents the latent heat of sublimation; De, effective diffusivity of water vapor through snow; t and T, temperature of the system in degrees centigrade and Kelvin, respectively. Values of De can be related to the mass flow rate of dry air, G as follows: De = 95.38 (G + 0.465 × 10−4)½. This relationship holds for snow densities from 0.376 to 0.472 g/cm3 and mass flow rates of dry air, G, from 2 to 35×10−4 g/cm2 sec. It is in complete agreement with the conclusion drawn by Yosida that under no-air-flow condition, the snow density has no effect on the extent of water vapor diffusion and thus the amount of heat transferred by this mechanism. The contribution of kv is about 7.5 per cent of the effective thermal conductivity, ke, when there is a mean snow temperature of −10°C and no air flow. However, for an air-flow rate of 10×10−4 g/cm2 sec, or for pore velocity of about 1.3 cm/sec, kv is about 19 per cent of ke. Therefore, the vapor transfer processes make a significant contribution to the process of heat transfer associated with a natural snow cover.